It has been common practice for many years to apply a tone signal with a voice transmission, which has a frequency associated with one or more receivers, so that only the receivers responsive to the particular frequency are operative to reproduce the voice transmission. Such a system is set forth and described in U.S. Pat. No. 2,918,571, issued Dec. 22, 1959 to Robert Peth, and assigned to the same assignee as the present application. The referenced prior art customarily utilize a vibrating reed device to establish the particular frequency transmitted, and a second vibrating reed device at the receiver to respond to the particular frequency transmitted. In order to conserve equipment, a single reed device has been used in a combined encoder-decoder device and such a system is described in U.S. Pat. No. 3,250,997, issued May 10, 1966 to William J. Cole and Robert H. Walker, and assigned to the same assignee as the present invention.
Although systems as are described in the above-referenced patents have been satisfactorily used for many years, such systems have the objection that they require a relatively large and expensive mechanical vibrating device. With the continuing trend toward reduction in size of electronic devices, and with the desire for elimination of mechanical devices, it has been proposed to use an active filter as the selective element to replace the vibrating reed devices previously used. It has been found, however, that various problems arise in connection with the use of an active filter in a combined encoder-decoder for use in the application which has been described. For example, in order to provide the high selectivity desired, the active filter must have a narrow bandwidth. This presents a problem in that substantial real time is required for the active filter oscillator to build up oscillations to the desired level. Another problem is that when the device is operating as an encoder producing oscillations, substantial energy is stored in the active filter and when the device is switched to decoder operation the energy remaining in the filter may produce a control signal, thereby falsely indicating that a signal of the desired frequency is received. Similarly, substantial energy remains in the filter when it is operating in the decoder mode, such that when the system is connected to operate as an encoder at the same frequency, the stored energy together with the energy building up in the oscillator circuit may cause overshoot.